= Solution
A <blackbody> absorbs all incident <electromagnetic radiation>, at every <frequency>, direction and polarization: its absorptivity is one. In <thermal equilibrium>, <Kirchhoff's law of thermal radiation> makes its <emissivity> one as well. Its emitted <spectrum> depends only on its absolute <temperature>, through <Planck's law>, rather than on the material of its surface.
A small aperture in a large isothermal cavity approximates a <blackbody> because radiation entering it undergoes many reflections and is very unlikely to escape before absorption. The equilibrium <radiation field> is isotropic and unpolarized. The total emitted <radiant exitance> is $\sigma T^4$ by the <Stefan–Boltzmann law>; the peak of the wavelength <spectrum> shifts to shorter <wavelengths> as <temperature> increases according to <Wien's displacement law>. An ideal absorber can therefore glow brightly when hot: “black” describes absorption, not the absence of thermal emission.
Back to article page
